The invention relates to voice message equipments for automatic exchanges, in particular those capable of offering a so-called "letterbox" service to the users of telephone sets connected to a digital time division exchange.
Voice message services are designed to allow the users of telephone sets to receive sound type messages over the telephone and to record them for subsequent consultation without having to answer calls when they are actually happening.
These services sometimes also enable the same users to transmit recorded telephone messages, which may may be personalized or otherwise, e.g. by broadcasts at selected times to specified called parties or to parties calling over the telephone system.
It is possible to provide such such services by individual telephone answering machine type equipments integrated in telephone terminals or sets, or located close by and connected thereto, but it is advantageous for such services to be provided by the exchanges to which the telephone terminals or sets are connected, with a set then having the option of being associated with an individual or shared "letterbox" both for receiving and for sending user messages.
This second technique enables nearly all of the hardware and software required to operate the system to be centralized in an exchange, while the telephone sets require few or no additions to be capable of operating the message service.
The voice message equipment of an exchange must therefore be capable of receiving and recording a plurality of messages simultaneously for the telephones which are connected thereto. This can be achieved, for example, by connecting the voice message equipment to the switching network of the exchange via at least one time division multiplex link over which messages received simultaneously are time multiplexed. All that is then required is a memory arrangement whose recording speed and capacity are compatible respectively with the bit rate and the volume of sampled speech signals transmitted over the multiplex link(s).
The voice message equipment of an exchange must also be capable of reconstituting any message intended for a telephone in real time and at a user's request, regardless of the total number of messages recorded and regardless of how old the message may be relative to the collection of messages simultaneously still in store.
This implies using very large capacity memories having short access times. For reasons of economy, it is known that hard disks may be used since they have large capacity and relatively short access time.